Use pcolormesh instead of imshow

[arafune]

The ARPES data is not just image data, coordinates is significant meanings.

Thus the pcolromesh should be used, rather than imshow.
The following files use the imshow. Refactor them by using pcolormesh.

• src/arpes/plotting/utils.py
• src/arpes/plotting/fermi_surface.py
• src/arpes/plotting/false_color.py
• src/arpes/plotting/spin.py

[arafune]

imshow_arr shoud be removed.